The invention relates to a new hydrocarbon conversion process wherein hydrocarbons are contacted with two reforming catalysts, one of which has a superior selectivity for dehydrocyclization.
Catalytic reforming is well known in the petroleum industry and refers to the treatment of naphtha fractions to improve the octane rating and/or to produce aromatic hydrocarbons for use as chemical feedstock. The more important hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization of normal paraffins to isoparaffins, dealkylation of alkylbenzenes, and hydrocracking. Hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane are to be minimized during reforming as this decreases the yield of gasoline boiling range products.
Catalysts comprising platinum, for example, platinum supported on alumina, are well known and widely used for reforming of naphthas and gasoline boiling range materials in order to produce high octane number gasolines.
A particularly advantageous method of reforming is in the presence of hydrogen with a catalyst composition of a porous solid catalyst support, such as alumina, and 0.1 to 3 percent platinum and 0.01 to 5 weight percent rhenium. Other bimetallic catalysts reported to be advantageous include platinum-tin, platinum-germanium, platinum-lead, and platinum-iridium.
The possibility of using carriers other than alumina has also been studied and it was proposed to use certain molecular sieves such as X and Y zeolites, because the pore sizes of the zeolites were large enough to pass the reactant and product molecules through the pores of the zeolite. However, catalysts based upon these molecular sieves have not been commercially successful.
In the conventional method of carrying out the aforementioned dehydrocyclization, hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of 430.degree. C. to 550.degree. C. and pressures of 100 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The catalysts hitherto used have given moderately satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction; can be used to improve the rate of conversion to aromatic hydrocarbons without requiring higher temperatures and lower pressures, which usually have a considerable adverse effect on the stability of the catalyst; and produce excellent results with C.sub.6 -C.sub.8 paraffins, but run length is a problem.
In one method of dehydrocyclizing aliphatic hydrocarbons, hydrocarbons are contacted in the presence of hydrogen at a temperature of 430.degree. C. to 550.degree. C. with a catalyst consisting essentially of a type L zeolite having exchangeable cations of which at least 90% are alkali metal ions selected from the group consisting of ions of sodium, lithium, potassium, rubidium and cesium and containing at least one metal selected from the group which consists of metals of Group VIII of the Periodic Table of Elements, tin and germanium, said metal or metals including at least one metal from Group VIII of said Periodic Table having a dehydrogenating effect, so as to convert at least part of the feedstock into aromatic hydrocarbons.
A particularly advantageous embodiment of this method is a platinum/alkali metal/type L zeolite catalyst because of its excellent activity and selectivity for converting hexanes and heptanes to aromatics, but run length remains a problem.